Some aspects of the late Quaternary geomorphology of the lower Manawatu Valley, New Zealand

The lower M 1nawatu Valley consists of three topographic units: the Tokomaru Marine Terrace, the Manawatu floodplain, and a coastal dune belt. The floodplain is underlain at depth by river gravels from which a date (NZ 3938B) of 42700 + 7650 − 3950 years B.P. was obtained. Many hox-shaped valleys have developed within the marine terrace adjacent to the eastern margin of the floodplain. Radiocarbon dating (NZ 3085B) of estuarine mollusc shells (6330 ± 70 years B.P.), and stratigraphic and morphologic studies provide evidence that the lower Manawatu and Oroua Valleys became estuaries during the Postglacial Transgression. During the latter stages of the transgression, two barriers, the Himatangi Anticline and Poroutawhao High, which lie parallel to the present coastline, restricted oceanic influences and facilitated the formation of the estuaries. Valleys cut in the marine terrace during the last glaciation, particularly those lying between Tokomaru and Levin, were partly filled with estuarine sediment to produce the flat valley floors characteristic of box-shaped valleys. The height and location of estuarine sediment, and the height of beach ridges near Otaki, indicate that there is no evidence in this area and adjacent areas to the south for a Holocene sea level higher than at present. Rates of uplift in the Levin-Shannon area (c. 0⋅35 m/10³ yr) are discussed and new evidence for the minimum age of the Levin Anticline (c. 60000 yrs) is presented.